The present generally concerns electrochemical fuel cells and more particularly to a method for fabricating bipolar plates.
Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells have intrinsic benefits and a wide range of applications due to their relatively low operating temperatures (room temperature to around 80° C., and up to 160° C. with high temperature membranes). The active portion of a PEM is a membrane sandwiched between an anode and a cathode layer. Fuel containing hydrogen is passed over the anode and oxygen (air) is passed over the cathode. The reactants, through the electrolyte (the membrane), react indirectly with each other generating an electrical voltage between the cathode and anode. Typical electrical potentials of PEM cells can range from 0.5 to 0.9 volts; the higher the voltage the greater the electrochemical efficiency. However, at lower cell voltages, the current density is higher but there is eventually a peak value in power density for a given set of operating conditions. The electrochemical reaction also generates heat and water as byproducts that must be extracted from the fuel cell, although the extracted heat can be used in a cogeneration mode, and the product water can be used for humidification of the membrane, cell cooling or dispersed to the environment.
Multiple cells are combined by stacking, interconnecting individual cells in electrical series. The voltage generated by the cell stack is effectively the sum of the individual cell voltages. There are designs that use multiple cells in parallel or in a combination series parallel connection. Separator plates (bipolar plates) are inserted between the cells to separate the anode reactant of one cell from the cathode reactant of the next cell. These separator plates are typically graphite based or metallic (with or without coating). To provide hydrogen to the anode and oxygen to the cathode without mixing, a system of fluid distribution and seals is required.
The dominant design at present in the fuel cell industry is to use fluid flow field plates with the flow fields machined, molded or otherwise impressed in the bipolar plates. An optimized bipolar plate has to fulfill a series of requirements: very good electrical and heat conductivity; gas tightness; corrosion resistance; low weight; and low cost. The bipolar plate design ensures good fluid distribution as well as the removal of product water and heat generated. Manifold design is also critical to uniformly distribute fluids between each separator/flow field plate.
There is an ongoing effort to innovate in order to increase the power density (reduce weight and volume) of fuel cell stacks, and to reduce material and assembly costs.
In a fuel cell system (stack & balance of plant), the stack is the dominant component of the fuel cell system weight and cost and the bipolar plates are the major component (both weight and volume) of the stack.
Bipolar plates are a significant factor in determining the gravimetric and volumetric power density of a fuel cell, typically accounting for 40 to 70% of the weight of a stack and almost all of the volume. For component developers, the challenge is therefore to reduce the weight, size and cost of the bipolar plate while maintaining the desired properties for high-performance operation.
The multiple roles of the bipolar plate and the challenging environment in which it operates means that the material from which it is made must possess a particular set of properties. The material should combine the following characteristics:
High electrical conductivity, especially in the through-plane direction;
Low contact resistance with the gas diffusion layer (GDL)—depending on the plate material and the thickness, the contact resistance with the GDL can be more important than the resistance of the plate itself;
Good thermal conductivity—efficient removal of heat from the electrodes is vital for maintaining an even temperature distribution;
Thermal Stability;
Gas impermeability to avoid potentially dangerous and performance-degrading leaks;
Good mechanical strength—so as to be physically robust and to support the MEA;
Corrosion resistance—bipolar plates operate in a warm, damp environment while simultaneously exposed to air and fuel over a range of electrical potentials (ideal conditions for corrosion to occur);
Resistance to ion-leaching—if metal ions are released from the plate they can displace protons in the membrane and lower the ionic conductivity;
Thin and lightweight while accommodating the flow channels and maintaining mechanical stability;
Low cost and ease of manufacturing;
Environmentally benign;
Recyclable.
A number of different methods have been used to manufacture bipolar plates including for example, U.S. Pat. No. 6,818,165 to Gallagher for “Method of Fabricating Fluid Flow Field Plates” on Nov. 16, 2004 and U.S. Pat. No. 6,997,696 to Davis et al for “Apparatus for Cutting Expanded Graphite Sheet Material” on Feb. 14, 2006. These methods, however, have a number of significant drawbacks. For example, the fabrication fluid flow field plates require four steps, namely roller embossing fluid flow channels; reciprocally embossing fluid distribution areas; die cutting manifold openings; and curing the plates in an autoclave. The methods used to roller emboss flow channels, reciprocally emboss fluid distribution manifolds, and then die cut the manifold openings requires careful alignment of the part between each of these steps. Additionally, four embossing dies and one cutting die per part are required, i.e., two mating roller dies, two mating reciprocal dies, and one cutting die to cut the manifolds. The methods use “pre-impregnated” expanded graphite which must be cured after part fabrication in an autoclave to improve the mechanical properties of the fluid flow field plate. The fluid distribution areas are not “straight” and therefore the roller is unable to emboss the entire part in one step.
Thus, there is a need for an improved method for fabricating bipolar plates.